Pneumatic rubber tires are composed of a plurality of components which conventionally include sidewalls which are subject to considerable flexing and scuffing during use of the tire, a phenomenon well known to those having skill in such art.
Sometimes a portion of such sidewalls are of a decorative white color in which a white colored titanium dioxide pigment is used to the exclusion of black colored carbon black reinforcing filler.
Sidewalls of pneumatic rubber tires may age somewhat prematurely as a result of one or more of weathering due to atmospheric conditions, fatigue cracking due to extensive flexing under operating conditions and abrasion due to scuffing.
Rubber tire sidewall aging due to weathering may be, for example, a result of exposure to one or more of ultraviolet light, ozone and high humidity. Antidegradants are conventionally mixed with the sidewall rubber to counteract such effects. Also, sometimes low unsaturation elastomers such as butyl rubber and EPDM's are included in the sidewall rubber composition to counteract such effects.
Rubber tire sidewall fatigue cracking is usually due to extensive flexing of the tire sidewall as the tire is run under load under service conditions. Such fatigue cracking may first initiate (crack initiation) and then propagate (crack propagation) over time. Oftentimes a cis 1,4-polyisoprene rubber is included in the tire sidewall rubber composition to counteract such effects.
It is important to appreciate that natural cis 1,4-polyisoprene rubber is susceptible to ozone attack and consequential damage such as visible surface cracking. Accordingly, amine-containing antiozonants are used in carbon black-containing, and therefore black-colored, tire sidewall rubber compositions which contain natural rubber, a practice well known to those having skill in such art.
Such amine-containing antiozonants are sometimes referred to as “staining antiozonants” because they tend to cause a discoloration of the tire sidewall rubber composition. Such discoloration is not particularly readily noticeable for carbon black-containing, black colored tire rubber sidewall composition, a phenomenon known to those having skill in such art.
However, in practice, for non-black colored tire sidewall rubber compositions, such staining antiozonants are not desired since they, in turn, conventionally promote a visible, unwanted discoloration of the non-black colored (e.g. white colored with titanium dioxide coloring pigment) tire sidewall rubber composition. Instead, phenol-based antiozonants are conventionally used for such non-black colored (e.g. white colored), natural rubber-containing tire sidewall rubber composition. However, phenol-based antiozonants are conventionally not as effective as amine-containing antiozonants for protecting the natural rubber from ozone attack as it is understood would be well known to those having skill in such art.
Therefore, there exists a need to create an ozone-resistant, non-discoloring tire sidewall rubber composition, particularly of a non-black colored (e.g. white colored with titanium dioxide pigment colorant) which does not contain amine-containing chemical antiozonants but which do contain the natural rubber.
Nanocomposites, as referenced herein, refer to composites comprised of at least one elastomer, particularly natural cis 1,4-polyisoprene rubber, which contains a dispersion of exfoliated clay platelets therein.
Historically, dispersions of exfoliated platelets of selective clays have heretofore been suggested for various thermoplastic and rubber compositions for use in various articles. For example, see U.S. Pat. Nos. 4,431,755, 5,840,796 as well as U.S. Pat. Nos. 6,087,016 and 6,232,389 and background U.S. Pat. Nos. 4,528,235, 4,536,425, 4,857,397, 4,911,218, 4,960,639, 4,983,432, 5,091,432, 5,049,609, 5,178,702 5,552,469, 5,576,372, 5,576,373 and 5,857,397.
Historically, exfoliated clay particles are formed by exfoliation of intercalated water swellable clay. Such intercalated clay may be prepared by, for example
(A) obtaining a clay which is comprised of a plurality of stacked layers, or platelets which is swellable upon its dispersion in water (e.g. smectite clay such as, for example, montmorillonite and hectorite clays as well as vermiculate clay),
(B) intercalating the clay by, for example, dispersing the clay in an aqueous solution containing a quaternary ammonium salt which causes the clay to swell by an ion exchange between the quaternary ammonium salt and ions within the galleries between the stacked platelets of the clay in a manner that the average spacing between the platelets expands from, for example about 10 Angstroms, to an average spacing in a range of, for example, about 20 to about 40 Angstroms, depending somewhat upon the quaternary ammonium salt used and the particular clay, followed by
(C) drying the treated, or intercalated, clay.
The intercalated clay may be exfoliated in a sense of at least a portion of individual platelets becoming separated from the intercalated clay itself to form exfoliated clay platelets. Therefore, such clay particles are usually in a form of exfoliated intercalated clay platelets of which a portion of the intercalated clay is in a from of exfoliated clay platelets (e.g. intercalated clay of which at least a portion has been exfoliated into exfoliated clay platelets).
A nanocomposite comprised of at least one elastomer and dispersion of exfoliated intercalated clay platelets may be thereby formed in which the clay itself is in a form of an intercalated clay which is at least partially exfoliated.
In one aspect, it may be desired that the exfoliated clay platelets, as well as the accompanying intercalated clay (intercalated, water swellable clay which accompanies clay platelets exfoliated from the intercalated clay, all contained as a dispersion in the elastomer composition), exhibit a significant coupling to the elastomer composition.
Such coupling the elastomer(s) of the elastomer composition rubber may be achieved, for example, by use of a rubber reactive quaternary ammonium salt that tends to couple the rubber reactive moiety of the exfoliated platelets and intercalated clay to the elastomer(s), particularly to diene-based elastomers.
Various water swellable clays composed of a stacked layers, or platelets, with galleries therebetween which contain the ion-exchangeable ions (e.g. cations comprised of one or more of sodium, potassium, lithium, magnesium, calcium, primarily sodium) are, for example, smectite clays such as for example montmorillonite and hectorite clays as well as vermiculite clay.
In the description of this invention, the term “phr” is used to designate parts by weight of a material per 100 parts by weight of elastomer. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” may be used interchangeably, as well as “unvulcanized” or “uncured”, unless otherwise indicated. The terms “compound” and “rubber composition” may be used interchangeably unless indicated. The term “carbon black” is used to refer to rubber reinforcing carbon blacks unless otherwise indicated. Exemplary rubber reinforcing carbon blacks may be referred to, for example, in The Vanderbilt Rubber Handbook (1987) on Pages 414 through 417.
The term “butyl-type” rubber as used herein refers to relatively unsaturated elastomeric copolymers of isobutylene such as, for examples, copolymers of isobutylene and a minor amount (e.g. from 0.5 to 3 percent) of a conjugated diene (e.g. isoprene) which are conventionally referred to as being a “butyl” rubber, halogenated (e.g. bromine or chlorine halogenated) butyl rubber which is conventionally referred to as being a “halobutyl”, “bromobutyl” or “chlorobutyl” rubber, as the case may be, and brominated copolymers of isobutylene and para-methylstyrene.
The term “EPDM” rubber, as used herein, refers to elastomeric copolymers of ethylene/propylene containing a minor amount (e.g. from about 2 to about 15 weight percent) of units derived from a non-conjugated diene such as, for example, ethylidene norbornene, 1,4-hexadiene and dicyclopentadiene, with an ethylene/propylene ratio in a range of from about 40/60 to about 80/20.